*{
    margin: 0;
    padding: 0;
}
body{
    min-width: 600px;
}

#left,#right{
    width: 200px;
    height: 200px;
    background: lightcoral;
}
#left{
    float: left;
}
#right{
    float: right;
}
#middle{
    height: 200px;
    background: lightgreen;
}